The ceiling is the most important surface in a room for the control of sound. If the ceiling is a hard, sound-reflecting material such as wood, plaster or concrete, sounds will spread throughout the room with little or no reduction; and noise levels will build up.
To minimize such build-up, it is common practice to make ceilings of sound absorbing material such as acoustical tiles. Such tiles are typically flat squares of porous material dimensioned to fit within the openings of a standard suspended ceiling structural grid. Because of their porous nature, the tiles are particularly effective for trapping and absorbing medium wavelength sound between about 1 kilohertz and 2 kilohertz, a narrow band of the audible sound spectrum.
While conventional planar acoustic ceiling systems work well in limiting medium wavelengths of the audible range, they leave much to be desired where there is substantial sound in the range between 4 kilohertz and 8 kilohertz and 100 hertz to 800 hertz. Such longer wavelength sound, encountered in many office and industrial applications, is not easily trapped or absorbed; and portions of it penetrate the planar ceiling system while other portions reflect from the ceiling in a coherent wavefront.
Anechoic chambers have been designed with relatively complex geometries for absorbing the full spectrum of sound (including inaudible wavelengths). These systems, however, are not generally suitable for economical manufacture, installation or use in commercial applications; and they are not suitable for retrofit into existing ceiling systems. Moreover, such totally non-reflective acoustic surfaces would be undesirable in occupied buildings.